Conventionally, a secondary cell (that is, a battery such as a nickel hydrogen battery or a lithium ion battery) for driving a motor in hybrid electric vehicles having a power generator for generating power by driving and regeneration by an engine and a motor which is operated with power from the battery for driving a driving wheel and in electric vehicles including the hybrid vehicles.
As an indication of a state of charge of the battery, a measure known as SOC (state of charge) is used, in which an SOC of 100% represents a fully charged state and an SOC of 0% represents a state in which the amount of charging is zero. In addition, there is a one-to-one correspondence between an open voltage Vocv and the SOC for a battery. Therefore, it is possible to calculate an SOC corresponding to an open voltage Vocv from a correlation between Vocv and SOC by measuring or estimating the open voltage Vocv of the battery.
Because the state of charge (SOC) of the battery changes depending on the running state of a vehicle (for example, starting, normal running, acceleration, deceleration, etc.) and electrical load of the vehicle (for example, brake lights, headlights, wipers, fan, etc.), the SOC must be estimated while the battery is being used. Conventionally, an SOC estimator in which the SOC is estimated by integrating a current (charging/discharging current) value of the battery is widely in use as an SOC estimator for the battery.
For example, Japanese Patent Laid-Open Publication No. 2000-166105 discloses a device as shown in FIGS. 14 and 15 in which, when a current sensor 16 for detecting a charging/discharging current of a battery 10 is normal (S400, S402), a charging/discharging current value detected by a battery ECU 34 is integrated to estimate the SOC (S404), and, when the current sensor 16 is abnormal, on the other hand, the state of charge of the battery is determined by the battery ECU 34 based on a battery voltage detected by a voltage detector 12 because the SOC cannot be detected through integration of charging/discharging current (S406, S407).
However, in the battery state-of-charge controller of Japanese Patent Laid-Open Publication No. 2000-166105, because the state of charge of the battery is determined based on the battery voltage from the voltage sensor when the current sensor is abnormal, the estimation precision of the determined SOC is low when the slope of the relationship between OCV and SOC changes.
In addition, there is a problem in that the estimation precision of the SOC is reduced in the SOC estimation unit through the integration method of the measured values in the current sensor when the measured current value is a value which contains an error or noise.